The biological utility of linear and cyclic synthetic peptides is dramatically circumscribed by their short half-lives in vivo and their lack of effectiveness when administered orally. These therapeutic disadvantages are primarily due to the extreme lability of biologically active peptides in the presence of the peptidases and proteases normally found in the digestive tract.
It is, therefore, desirable to stabilize the biologically active peptides against destructive enzymes, such as proteolytic enzymes, in order to improve the pharmacokinetic properties of these peptides. Enhanced stability to enzymatic degradation would also make such peptides more useful as therapeutic agents.
One way to stabilize peptides is to stabilize their backbone amide linkages. Recent advances in chemical replacement and modification of peptide linkages indicate that such linkage stabilization is feasible. In one method, replacement of peptide linkages at positions amenable to peptidase and protease cleavage with thioamide bonds produces analogues that are more stable to enzymatic degradation.
These analogues also display enhanced pharmacological activity. See, for example, G. Lajoie et al., "Synthesis and Biological Activity of Monothionated Analogs of Leucine-enkephalin", Int. J. Pept. Protein Res., 24, p. 316, (1984). Thiopeptide derivatives have also demonstrated increased activity in vivo as biological response modifiers, neuroeffectors, and immunomodulators, as compared with their oxygenated analogs. K. Clausen et. al., "Evidence of a Peptide Backbone Contribution Toward Selective Receptor Recognition for Leucine Enkephalin Thioamide Analogs", Biochem. Biophys Res Commun., 120, p. 305, (1984).
One method for forming a thioamide bond involves replacing the carbonyl oxygen of the native peptide bond with a sulphur, See, for example, K. Clausen et. al., "Studies on Amino Acids and Peptides Part 6. Methods for Introducing Thioamide Bonds into the Peptide Backbone: Synthesis of the Four Monothio Analogues of Leucine Enkephalin", J. Chem. Soc. Perkin Trans., pp. 785-98 (1984) which describes a method of thioacylation using dithioesters to replace the carbonyl oxygen atom with a sulfur atom.
However, the known thioacylation methods suffer from several disadvantages. First, the syntheses of the prior art thioacylating reagents are cumbersome and difficult. Furthermore, these syntheses produce low overall yields of the desired thiopeptide and often produce significant amounts of undesired and difficult to remove by-products. The prior art syntheses are also disadvantageous in that they can only be carried out on a small scale due to difficult purification schemes and extremely toxic reagents.
Further, the optical integrity of thiopeptides produced by these prior art methods is often not maintained. This further reduces the potential use of the thiopeptides as pharmacological agents.
Finally, the prior art thioacylation methods are limited to the production of linear thiopeptides. They deliver a single thionated amino acid to the N-terminus a growing linear peptide chain. Accordingly, these methods cannot be used to improve the stability and biological activity of known cyclic peptides by replacing peptide bonds in them with thioamide bonds.
For the reasons recited above, there is a need for a thioacylating reagent which is capable of catalyzing the formation of both linear and cyclic thiopeptides. In addition, there is a need for a thioacylating process which can be run on a large scale while producing thiopeptides in high overall purity and yield. There is also a need for a thioacylating process which will retain the optical integrity of the product thiopeptides. There is also a need for cyclic thiopeptides which demonstrate superior biologically useful characteristics, such as increased resistance to enzymatic degradation and improved pharmacological activity.